IB Physics SL A.2.20 Elastic & inelastic collisions

Practise deciding whether collisions are elastic or inelastic by comparing momentum and kinetic energy before and after impact.

Exam points

use momentum conservation to find post-collision speeds, then compare total kinetic energy before and after
show a collision is elastic by calculating equal total kinetic energy on both sides of the impact
predict outcomes when objects stick together, including reduced speed and a loss of kinetic energy

[Maximum number: 2]

B3. This question is in two parts. Part 1 is about a collision. Part 2 is about electric current and resistance.
Part 1 A collision
Two identical blocks of mass 0.17 kg and length 0.050 m are travelling towards each other along a straight line through their centres as shown below. Assume that the surface is frictionless.

Figure for Question A.2.20—Elastic and inelastic collisions question 1 — IB Physics SL

The initial distance between the centres of the blocks is 0.900 m and both blocks are moving at a speed of $0.18 \mathrm{~m} \mathrm{~s}^{-1}$ relative to the surface.

As a result of the collision, the blocks reverse their direction of motion and travel at the same speed as each other. During the collision, 20 % of the kinetic energy of the blocks is given off as thermal energy to the surroundings.

State and explain whether the collision is elastic or inelastic.

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A.2.20—Elastic and inelastic collisions question 1